Basin sill depth and episodic flushing influence bathypelagic nekton communities and goose-beaked whale foraging in the Southern California Bight

<title>Abstract</title> <p> Basin-scale (5–50 km) hydrographic variability within the Southern California Bight (SCB) structures bathypelagic (&gt;1000 m depth) mid-trophic nekton communities and the prey community available to deep-diving predators such as goose-beaked whales. At three mooring sites, we combine multi-year time series of hydrography, acoustic backscatter, and passive acoustics with environmental DNA (eDNA) from conductivity-temperature-depth (CTD) casts to resolve fine-scale spatiotemporal variability (hours to seasons) in water mass composition, bathypelagic nekton distribution, and beaked whale echolocation clicks at ~1200 m depth. Despite narrow temperature (&lt;0.67°C) and salinity (&lt;0.056 g kg ^-1 ) ranges, persistent differences between basin hydrography corresponded with distinct nekton assemblages: acoustic backscatter composition and cephalopod eDNA composition both indicated site-specific communities differentiated by dissolved oxygen (DO). Internal waves redistributed existing nekton patches at tidal timescales, while larger-scale physical forcings modulated total nekton backscatter over seasonal to interannual periods. In one case, an episodic basin flushing event coincided with increased nekton backscatter and a subsequent increase in goose-beaked whale acoustic detections. Collectively, these findings suggest that the SCB is a mosaic of heterogeneous bathypelagic habitats, with basin bathymetry and frequency of deep-water exchange jointly determining mid-trophic nekton community structure. </p>